A power source circuit for producing a plurality of different level voltages from an output voltage of a battery (hereinafter referred to as “battery voltage”) has been proposed for use in a battery-driven electrical equipment. Japanese Patent Application Publication No. HEI-6-54457 discloses a power source circuit including a DC-to-DC converter, a series-connection voltage adjusting unit, a voltage detector, and a change-over device. The DC-to-DC converter operates to boost the battery voltage to a first voltage (for example, 12V). The series-connection voltage adjusting unit operates to selectively lower the battery voltage or the first voltage whichever is applied thereto to a second voltage (for example, 5V). The voltage detector operates to detect the battery voltage. The change-over device changes over the voltage applied to the series-connection voltage adjusting unit to either the battery voltage or the first voltage. The voltage detector operates the change-over device to change the voltage applied to the series-connection adjusting unit to be the battery voltage when the battery voltage is larger than a predetermined level, whereas the voltage detector operates the change-over device to change the voltage applied to the series-connection adjusting unit to be the first voltage when the battery voltage is equal to or smaller than the predetermined level.